The invention can be applied in various contexts, and will be explained first of all in connection with the context in which this sensor turns out to be particularly advantageous. This context is that of measuring the speed of a motor vehicle by optically observing the passing of the ground below the vehicle: the sensor is mounted under the vehicle.
Speed sensors for driving vehicles are most often based on measuring the speed of rotation of the wheels, but they are not very accurate because there is always uncertainty about the diameter of the wheels (depending on the wear and the level of inflation of the tires) and uncertainty about whether their rotation is without slippage: skidding and the lack of grip of the wheels are the cause of slippage that disturbs the measurement. In addition, these sensors are slow, since the speed is available only by integrating pulses and the pulses have a very low repetition rate when the vehicle is being driven slowly.
This is why it has already been proposed to attach optical sensors underneath the vehicle, which observe the ground moving relative to the vehicle and deduce from this the true speed of the vehicle. The ground always has a high-contrast optical texture on account of the roughness of the surfacing (tar, for example). A photosensitive element that observes a small area of the road illuminated by a light source will, if the vehicle moves forward, see the light flux varying as a function of this texture. By making successive measurements of correlation between portions of the signals received by two identical sensors offset longitudinally in the run direction of the vehicle and by thus observing the same portion of ground at different instants, it is possible to find an optimal correlation allowing the speed of the vehicle relative to the ground to be deduced if the distance between the sensors is known.
One simple solution consists in projecting a spot of light onto the surface of the road using a beam of light directed perpendicularly to the latter and in observing the light reflected or scattered in the direction of the source. A difficulty then occurs when the reflectance of the ground is too high: the sensor is dazzled by the specular reflection of the light on a plane reflecting surface and the observed image is unusable. This is the case notably when the road is wet: even if the specularly reflected intensity is only 3% of the intensity emitted by the source, this is still too much for the sensor, which must be able to detect very weak illuminations backscattered by ground that is not wet or the ground below the layer of water.
To solve this problem signal-processing-based solutions, on the one hand, have already been proposed, for example to eliminate that portion of the signal which is not coming from the observed ground texture but which is coming from the layer of water covering said texture. These solutions are not very effective due to the highly variable nature of the conditions observed.
The use of optical isolators has also been proposed, which, starting with the light polarization conditions (these being different for the specular reflection produced by the water and for the backscattering produced by the ground), allow the undesirable reflected light to be removed. These isolators (polarization splitters) generate very significant intensity losses in the signal so that they are difficult to use.
Another solution consists in tilting the direction of light emission relative to the surface of the ground. The receiver observes the ground via optics with their axes normal to the ground, but with the source illuminating the ground obliquely. If there is any water, the specularly reflected rays are also oblique and do not reach the receiver. Only the rays backscattered vertically by the ground are observed by the receiver. This technique of eliminating the specular reflection by tilting the axis of illumination relative to the axis of observation is similar to the technique of darkfield lighting used in microscopy.
However, it has been noted that this darkfield lighting technique is not satisfactory in the envisioned application of measuring the speed of movement of a vehicle relative to the ground. One drawback arises when the distance between the sensor and the ground is not constant. If the distance is too large or too small in relation to the theoretical distance where observation is optimal, then the spot of light produced by the source on an axis tilted to the ground is no longer beneath the receiver, and the receiver optics, having a relatively narrow receiver aperture, will no longer “see” the spot of light. Alternatively it is then necessary to enlarge greatly the size of the light spot and the subsequent correlation becomes much more difficult, and in addition the power of the light source must be enhanced accordingly. Consequently, this solution is not acceptable in contexts where the distance is not fixed. This is indeed what happens in the case of a vehicle on the ground.
European patent EP 0562924 describes such a solution: oblique illumination, normal observation.
Of course, the principle remains the same, with the same drawbacks, if the ground is illuminated normally while tilting the axis of observation of the receiver.
Means of analyzing the movement of an optical mouse of a computer, using an image sensor and a spatial correlation of successive images detected by the sensor as the mouse moves, have also been proposed in the prior art (US2003/0034959). Finding the direction of image shift is done by finding the best correlation among all the possible directions of movement.